In order to implement Windows Sockets purely as a DLL, it may be necessary for the DLL to post messages internally for communication and timing. This is perfectly legal; however, a Windows Sockets DLL must not post messages to a window handle opened by a client application except for those messages requested by the application. A Windows Sockets DLL that needs to use messages for its own purposes must open a hidden window and post any necessary messages to the handle for that window.